1. Field of the Invention
The present invention generally relates to ventilation systems for buildings, and more particularly relates to methods and systems for providing good quality conditioned air to occupied building spaces.
2. Background and Prior Art
Air-conditioning manufacturers, architects, and professional design engineers have expended huge efforts in optimizing the design of building air-conditioning and ventilation systems. Annual sales of equipment amount to tens of billions of dollars and annual energy costs for heating and cooling have similar magnitudes. In addition, the costs associated with reduced productivity of workers because of uncomfortable environmental conditions may be several times these figures, although such consequential costs are difficult to quantify. Yet despite these efforts at optimization the fundamental principles for ventilating and conditioning the air in buildings have remained essentially the same since the introduction of the first air conditioners in the 1920s. Conventional approaches to air conditioning have inherent problems that severely limit their efficiency, raise installed cost, and frequently produce poor environmental comfort conditions in the building space. Solving these problems requires major changes in the basic configuration of air-conditioning systems.
Conventional air-conditioning systems use a relatively small volume of air for cooling. The typical arrangement uses a vapor-compression refrigeration system to cool a mixture of return air and outside air to approximately 55.degree. F. and then distribute the cooled air through ducts to the building space. The low supply air temperatures are used because of the need to cool the air below its dew point to remove moisture. The low air temperatures are also necessary to meet the sensible cooling needs of the space without using excessively large ducts.
There are several significant problems with this approach. The first relates to fan or blower energy consumption. Because air in the conventional systems flows through relatively restrictive ductwork, fan static pressures are quite high. Typical pressures range from less than 0.5 inches of water for residential systems to as much as 5 to 10 inches of water for large commercial cooling systems. These high static pressures result in large energy consumption by the fan, and also add to the cooling load for the rest of the system. In many commercial systems, the heat generated by fan operation accounts for as much as 20 to 30 percent of the total cooling load for the building. The net result is a very inefficient cooling system.
A second problem pertains to the high compressor energy required. The required low air supply temperatures dictate even lower evaporating temperatures, typically 40.degree. to 50.degree. F. for the compressor system. Such low evaporating temperatures necessitate increased work for the compressor which further reduces the efficiency of the system.
A third problem with the conventional air conditioning system is poor indoor air quality associated with high duct humidity. Conditions over 70% relative humidity allow the growth of mold and fungus in ductwork. The relative humidity in the supply ducts for conventional systems is frequently over 90%. In addition, water from wet coils drips onto drain pans and can also wet nearby ductwork. These wet conditions create potential breeding grounds for many types of microbes that can cause health, respiratory, and odor problems.
A fourth shortcoming with conventional systems is the high noise levels emitted. The high static pressure caused by restrictive ductwork creates a need for a powerful fan that usually is quite noisy. In addition, metal ducts are notorious noise transmitters. Common fixes for the noise problem include the use of fiberglass duct liners. Unfortunately these liners increase cost and pressure drop and also can contribute to problems with molds given the high relative humidity in most ducts.
A fifth problem is the potential for drafts with conventional cooling systems. The low air supply temperatures and high velocities create the possibility of extremely uncomfortable conditions near the vents. Designers must take special care to ensure adequate mixing of room air and supply air to reduce drafts to acceptable levels.
A sixth problem is the need for simultaneous heating and cooling. Most office buildings have a single air handling system for the interior and exterior zones. In cold weather the interior zones still need cooling because of heat from people, lights, equipment, etc., while the exterior zones need heat. The usual solution is to supply cool air to the entire building in order to satisfy the cooling needs of the interior, while perimeter heaters or local heaters in the ducts servicing the exterior zones provide the heat necessary to satisfy the heating load and overcome the cooling from the supply air.
A major objective of the present invention is thus to improve energy efficiency and to reduce or eliminate the problems associated with existing conventional air conditioning systems discussed above.